The present invention relates generally to an optical signal transmission using an optical fiber and more particularly to an optical transmitter as well as an optical signal transmission system which are suited advantageously for digital optical communication.
Progress in the optical communication technology in recent years is promising realization of a high-speed long-haul signal transmission. However, when an optical signal of high intensity is transmitted over a long distance through an optical fiber which scarcely attenuates the signal, there arise undesirable phenomena such as distortion of signal waveform, rapid or abrupt attenuation of the signal and the like due to waveform dispersion, non-linear optical effects and other causes, which phenomena can no more be neglected.
In order to protect the waveform against degradation or deterioration due to the waveform dispersion, it is necessary to suppress spectrum spreading (i.e., so-called chirping) of the optical signal in the optical transmitter. In this conjunction, an external modulation system (i.e., a system for modulating a light beam of a constant intensity or power emitted from a laser with an optical intensity modulator) is now replacing a hitherto known direct modulation system (i.e., a system for modulating the intensity of light beam by modulating a current for driving the laser). This is primarily for the reason that the external modulation system is less susceptible to the chirping. However, even when the external modulation system is adopted, there takes place equivalently a chirping due to phase modulation or shift which occurs in accompanying the intensity modulation. Under the circumstances, approaches or measures for lowering the phase modulation efficiency relative to the intensity modulation efficiency are now energetically studied.
On the other hand, stimulated Brillouin scattering which is one of the non-linear optical effects is more likely to take place as spectrum spreading of the light source becomes smaller. In reality, when an optical signal of high intensity is inputted to an optical fiber for long-haul transmission, the stimulated Brillouin scattering takes place, incurring rapid attenuation of the light signal propagating or travelling through the optical fiber. At this juncture, it is noted that the measures for coping with the stimulated Brillouin scattering due to the spectrum spreading is naturally in conflict with the measures for reducing the chirping of the light source. Accordingly, there exists a great demand for a method or technology which can solve both the problems mentioned above simultaneously in a satisfactory manner.
An approach for dealing with the problem of stimulated Brillouin scattering in the optical communication is discussed in "IEICE (The Institute of Electronics, Information and Communication Engineers) Technical Report OQE 91-114, OCS 91-49", p. 75. FIG. 2 shows a structure of an optical transmitter implemented on the basis of the technology disclosed in the above-mentioned literature. Referring to the figure, a light beam emitted from a light source 1 for signal transmission undergoes intensity modulation in an intensity modulator 2 in accordance with a signal to be transmitted (hereinafter referred to also as the transmit signal or transmission signal) generated by a transmit signal generator 4 before being transmitted through an optical fiber. As the measure for suppressing the stimulated Brillouin scattering, direct frequency modulation is effected for the light source 1 with a signal generated by a signal generator 5 for thereby spreading the spectrum of the light beam emitted from the light source 1.
The optical signal having spread spectrum suffers from jitter under the influence of dispersion which the optical signal undergoes in the course of propagation through the optical fiber. According to the teaching disclosed in the aforementioned literature, the time jitter can be estimated smaller than 14 ps when the modulating frequency is set higher than 10 kHz with frequency deviation being within 1 GHz for a transmission fiber exhibiting a total dispersion of 1800 ps/nm.
The prior art technique described above suffers from problems that a driver circuit of complicated configuration is required for the direct frequency modulation of the light source in order to spread the spectrum of the light beam for signal transmission and that variation in the light intensity can not be avoided.
Further, in the literature mentioned above, no discussion is made concerning the method of compensating for the phase-shifted components as a method of solving the problem of chirping brought about by the intensity modulator.